This invention relates to methods, compositions, and apparatuses for the generation of chlorine dioxide (ClO2). Chlorine dioxide is a commonly used biocide to treat various water process systems and has effectively replaced more traditional chlorine biocide. Unlike chlorine, chlorine dioxide remains a gas when dissolved in aqueous solutions and does not ionize to form weak acids. As a result, chlorine dioxide is effective over a wide pH range, and is a logical choice for water process systems that operate at alkaline pH or that have poor pH control. Moreover, chlorine dioxide is a highly effective microbiocide at concentrations as low as 0.1 parts per million (ppm) over a wide pH range.
The biocidal activity of chlorine dioxide is believed to be due to its ability to penetrate microbial cell walls and react with essential amino acids within the cell cytoplasm to disrupt cell metabolism. This mechanism is more efficient than other oxidizers that only “burn” on contact with the cell wall surfaces and is therefore more effective against legionella, algae and amoebae cysts, giardia cysts, coliforms, salmonella, shigella, and cryptosporidium.
Unfortunately, chlorine dioxide in solution is unstable with an extremely short shelf life and thus, is difficult to transport or store. Also various laws limit or prevent the transportation of chlorine dioxide. As a result, chlorine dioxide must typically be generated at its point of use.
It is known to produce chlorine dioxide electrolytically by the electro-oxidation of chlorite ions. U.S. Pat. No. 2,163,793 describes an electrochemical chlorine dioxide generating process in which an aqueous solution of alkali metal chlorite and alkali metal chloride is electrolyzed in an electrolytic cell equipped with a porous diaphragm separating the anode and the cathode compartments.
British Patent No. 714,828 describes a process for the production of chlorine dioxide by electrolyzing an aqueous solution containing chlorite and a water-soluble salt of an inorganic oxy-acid other than sulfuric acid. U.S. Pat. No. 2,717,237 discloses a method for producing chlorine dioxide by electrolysis of chlorite in the presence of a water-soluble alkali metal sulfate (e.g., sodium sulfate). Japanese Patent Publication 81-158883, published Dec. 7, 1981, describes an electrolytic process for producing chlorine dioxide by electrolysis of chlorite in which the electrolyzed solution, at a pH of 2 or less, is fed to a stripping tank where air is introduced to recover the chlorine dioxide.
U.S. Pat. No. 4,542,008 describes an electrolytic process for chlorine dioxide production in which the sodium chlorite concentration of the solution leaving the anode compartment is measured by means of a photometric cell.
PCT Published International Patent Application WO 94/26670 discloses a method of producing chlorine dioxide from sodium chlorite in which the gaseous product along with the water vapor is removed from the electrolyzed solution by means of a microporous, hydrophobic gas membrane. By removing water at the rate of its input to the anolyte, a continuous, environmentally innocuous operation with no undesired effluent can be effected.
Another method of producing chlorous acid is through the use of ion-exchange resin beds. Ion exchange is the reversible interchange of ions between a solid (ion exchange material) and a liquid in which there is no permanent change in the structure of the solid. By contacting a resin with an excess of electrolyte, the resin can be converted entirely into a desired salt form. The ion exchange process involves diffusion through a film of solution that is in close contact with the resins and diffusion within the resin particle. The manufacture of ion exchange resins can involve the preparation of a cross-linked bead copolymer either as cation exchanging resins, or as anion exchanging resins. Cation exchange resins are constructed and arranged to retain cations present within the liquid.
Published PCT International Patent Application WO 91/09158 and the corresponding U.S. Pat. No. 5,106,465 disclose a method of producing chlorine dioxide from alkali metal chlorite in an ion exchange compartment of a multi-compartment cell in which hydrogen ions generated in the anode compartment enter the ion exchange compartment through a cation exchange membrane, causing chlorite ion decomposition and forming chlorine dioxide.
In each of these methods however, either an expensive catalyst is required for optimal chlorine generation or the reagents are incompatible and present a dangerous or hazardous condition if inadvertently mixed during operation. As a result, there remains need for improved methods, compositions, and apparatuses for the generation of chlorine dioxide.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “Prior Art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR § 1.56(a) exists.